1. Field of the Invention
The present invention relates to a system for enlargedly observing an image. More particularly, the present invention relates to a system for enlargedly observing an image which is displayed on an image display screen having a plurality of pixels arranged two-dimensionally with a periodicity as a CRT (Cathode Ray Tube) display or a liquid crystal display, using a thin transparent lens that is configured to enlargedly and transparently observe an image of which a plurality of image enlarging optical elements are arranged two-dimensionally with a periodicity by a predetermined pitch. The present invention also relates to a thin lens apparatus for use in such system for enlargedly observing an image. Further, the present invention relates to a filter assembly for arranging a filter in front of a display device.
2. Description of the Related Art
As a thin (low-profile) lens having a function of enlarging an image, a Fresnel lens is well known. The Fresnel lens has a structure of a large number of the prism elements in ring shapes, of which an outer surface thereof forms the parts of the curved surface of the lens, which are arranged concentrically, and can be made much thinner than an ordinal lens.
Similarly, as a thin lens having an image enlarging function, there is a lens beads array of which a large number of small size convex lens elements are arranged two-dimensionally on one plane.
Furthermore, a lenticular lens of which a large number of linear image enlarging optical elements are arranged in parallel is also known, and a thin lens enabling to evenly enlarge the image into both directions of the length-width wise can be formed by putting two lenticular lenses together such that the image enlarging optical elements thereof are intersecting at right angles each other.
The display screen can be enlargedly observed by placing the thin lens of the type described above in front of an image display device of an image display apparatus such as a CRT display or a liquid crystal display.
However, since the image display apparatus such as the CRT display or the liquid crystal display is that, on the screen thereof, a plurality of pixels are arranged two-dimensionally with a periodicity, and also in the thin lens described above the image enlarging optical elements are arranged two-dimensionally with a periodicity, when observing the screen of the image display apparatus of this kind through the thin lens described above, a clear image can not be obtained because of the Moire interference fringes being resulted from a diffraction phenomena, thereby causing a trouble in the image observation.
In convention, it is well known to place a filter in front of the display device comprising the CRT display or the liquid crystal display in order to ease the eyestrain caused by watching the display screen intensely.
A surface of the filter eases of the eyestrain and implements an easy-to-see screen by making a reflection prevent treatment of a hard-coat by the existence technique thereon, and further adding an anti-glare treatment, and a coloring treatment and the like thereto.
A conventional filter to be used for this purpose has a structure of which a filter is engaged in a frame formed by a metal wile or a plastic material, and is configured such that the filter is placed in front of the display screen by hanging the frame of which the filter is engaged in on the display device.
The inventors of the present invention have been proposed to place a Fresnel lens in front of the display screen in the display device of this kind, as in the Japanese Patent Application No. 9-3607. Herein, the purpose of using the Fresnel lens is to enhance a resolving power of the display by the CRT, and to facilitate to see a screen with an diffusion effect by the Fresnel lens. In this case, in order to achieve a required effect, it is desirable that an optical center of the lens is to be located around a center of the display screen, in a view point of the optical characteristics of the lens.
However, in the conventional filter assembly designed for the display device, a positioning precision of the filter is not sufficient, and also it was not easy to make a positioning adjustment. Accordingly, when using the Fresnel lens as a filter of the filer assembly, there was a problem that a performance of the Fresnel lens can not be demonstrated sufficiently.
Further, although it is desirable to enable to adjust a position of the filter in accordance with the designed dimensions of the frame in the display device when using a conventional filter without using the Fresnel lens, the conventional filter assembly can not comply with such a demand.